1. Field of the Invention
This invention relates to the production of substantially spherical metal hydroxide particles under turbulent flow conditions. More particularly, this invention relates to the production of tailor-made particle size distributions of such particles by vapor phase hydrolysis of an aerosol which contains one or more metal alkoxides.
2. Background Art
Metal oxide and metal hydroxides are useful in the formation of ceramic materials, paint pigments, fire retardants and the like. Of particular interest in the formation of ceramics are spherical, non-agglomerated particles of substantially uniform size because of the possible reduction of sintering time and temperature while increasing mechanical strength. When such particles are of high purity, they are especially useful in the production of ceramic substrates for the semiconductor industry where even trace impurities of certain materials, e.g. heavy metal oxides, such as thorium oxide which emits alpha particles, are highly undesirable.
Methods for the production of substantially spherical metal oxides and hydroxides from the hydrolysis of aerosol formed from metal alkoxides are known. For example, Visca et al in "Preparation of Uniform Colloidal Dispersions of Chemical Reactions in Aerosols: 1. Spherical Particles of Titanium Dioxide", published in the Journal of Colloid and Interface Science, Vol. 68, No. 2, February, 1979, at pp. 308-319, discussed the portion of TiO.sub.2 spheres of narrow size distribution using titanium alkoxide and TiCl.sub.4 starting materials in which the starting material is first evaporated into a gaseous carrier and then condensed onto AgCl seed nuclei in a condensation zone to form an aerosol dispersed in the gas. The aerosol is then hydrolyzed by contact with water vapor to form the spherical particles of TiO.sub.2. A similar process is described by the authors in Matijevic et al U.S. Pat. No. 4,241,042.
Ingebrethsen et al in "Particles of Uniform Colloidal Dispersions by Chemical Reactions--2. Spherical Particles of Aluminum Hydrous Oxide", Journal of Aerosol Science, Vol. 11, Pergamon Press, 1980, Great Britain, at pp. 271-280, describe the formation of uniform colloidal aluminum hydrous oxide aerosols by hydrolyzing droplets of aluminum sec-butoxide. The use of AgCl nucleating material is again described. Ingebrethsen et al particularly caution about preserving the laminar flow in the hydrolysis zone, indicating, on page 273, that turbulence in the flow caused a broadening of the size distribution of their final solid particles.
The deleterious effect of turbulence in the hydrolysis zone is also discussed in European Patent No. 117,755 where the use of turbulent flow is employed to initially form the aerosol. The turbulent flow is used by the patentees in the aerosol generating stage to form the aerosol by mixing an inert gas stream containing vapor of a hydrolyzable metal compound with a cold inert gas stream. By mixing the two streams in a turbulent flow with a Reynolds number equal to or higher than 1800, an aerosol can apparently be formed without requiring the generation of solid nuclei. The patentees, however, warn that laminar flow must be reestablished in the subsequent hydrolysis zone to reduce the risk of coalescence of the droplets coming from the previous aerosol forming stages.
Formation of an aerosol without the use of seed nuclei is found in the aforementioned Matijevic U.S. Patent (and its U.K. counterpart patent, GB No. 2070579B) at column 3, line 14, to column 4, line 3, under a discussion of homogeneous and heterogeneous nucleation. Conditions of supersaturation are used to produce the self-induced or spontaneous nucleation referred to as homogeneous. However, the patentees state that homogeneous nucleation is more sensitive to temperature, pressure and velocity gradients in a flow system, which results in concentration gradients. Moreover, homogeneous nucleation is said to be more easily affected by contaminants and irregularities in container surfaces. As a consequence, the patentees conclude, it is more difficult to control the particle size in a homogeneous nucleation system.
It is also known to produce such spherical particles under laminar flow conditions from mixtures containing more than one metal alkoxide. Ingebrethsen et al in an article entitled "Kinetics of Hydrolysis of Metal Alkoxide Aerosol Droplets in the Presence of Water Vapor" published in the Journal of Colloid and Interface Science, Volume 100, No. 1, July 1984, describe the hydrolysis of aluminum sec-butoxide, titanium ethoxide, and a mixture thereof at Reynolds numbers measured at 74, 50 and 48 in various regions of the apparatus, i.e., under laminar flow conditions.
It would, however, be desirable to be able to produce substantially spherical metal hydroxide or oxide particles having tailor-made particle size distribution of unimodal, bimodal and trimodal sizes of either single hydroxides or mixtures of hydroxides by the hydrolysis of an aerosol formed from metal alkoxide vapor. Particular particle size distributions, for example, are useful in achieving desired physical properties in a ceramic made therefrom while particular blends of metal oxides in ceramics can have utility in electrical applications by changing the resistivity and dielectric properties of the material.